1. Field of the Invention
The present invention relates to an electro-acoustic transducer of the loudspeaker-type comprising a membrane which is flexible or rigid before assembling and of which the front portion has an aperture with a predetermined half-angle .alpha. and is the site of vibrations responsible for sound emission which are transmitted in the material of the membrane from the motor of the transducer with a velocity V.sub.m such as V.sub.m #V.sub.0 /cos .alpha., where V.sub.0 is the velocity of sound waves in air.
2. Description of the Prior Art
In the conventional loudspeakers, the membrane is the seat of standing waves depending upon the frequency to be reproduced. For certain frequencies, the position of nodes and antinodes involves a maximum acoustic radiation in conjunction with resonances and a tone colouration. For other frequencies, the membrane behaviour is inverse and the acoustic radiation is minimum. In this case, if it is considered that the membrane is excited with a same energy, the amplitude of the membrane displacement increases as a total loss; the acoustic radiation impedance becomes wholly reactive. This leads to a low sound output and amplitude distorsions. In this type of loudspeaker, if the excitation in red noise ceases, the energy stored by the membrane is dissipated non-uniformly as a function of the frequency; the loudspeaker displays an acoustic tailing for certain frequencies.
The above-mentioned drawbacks have for consequence an amplitude-frequency response curve undulating at least in the high frequency range of its transmission band and the phase curve ceases to be at minimum. The sound power output of the loudspeaker is relatively low, because a considerable proportion of stored energy in the membrane is not radiated acoustically but dissipated in the form of heat, in particular by negative electrical feedback in the moving voice coil.
Moreover, in the known loudspeakers it is necessary to achieve a compromise between the flexibility of the membrane suspension, the choice of membrane material and the mass of the movable assembly of the motor, on one hand, and the width of the frequency range to be reproduced by the loudspeaker and its electro-acoustical output on the other hand. For the reproduction of frequencies up to medium frequencies, the membrane suspension is generally flexible and the moving emitting mass is large, in order to enable the membrane to move with a large amplitude. For reproducing medium to high audible frequencies, the membrane may by contrast be mounted rather rigidly on the loudspeaker chassis and be of small dimensions, because only the membrane area to the apex of the cone vibrates. Associated with the requirements in this second case, the mass of the movable assembly of the motor must be low, because the vibrational velocity in the membrane becomes high. In order to reproduce sound waves without non-linear amplitude distortions within a wide frequency range, it appears that it is necessary to constrain the geometrical deformations of the membrane to be much smaller than the amplitudes responsible for acoustic radiation, in analogy with the operation for which the membrane moves as an ideal rigid acoustic piston radiator, which is certainly not achieved, at least above a pulsation .omega..sub.0 =2 V.sub.0 /a, where V.sub.0 is the velocity of sound in air and a is the radius of the emitting portion of the loudspeaker. Consequently, the construction of a loudspeaker capable of reproducing a wide frequency range is difficult to achieve. This also results from the fact that sound waves generated at any two points along a generatrix of the conical membrane are not in phase at any listening point and that this phase shift is all the more pronounced as the frequencies to be reproduced become higher.
In order to avoid the above-mentioned drawbacks, Lincoln Walsh teaches in U.S. Pat. No. 3,424,873 a loudspeaker such as defined in the first paragraph of the present description. According to this patent, the membrane of the loudspeaker is rigid and is made in glass fibre, paper or aluminium and moreover, has a conventional conical shape.
Operating of this loudspeaker is founded on the fact that, in order to obtain sound waves in phase at any listening point, the vibration velocity V.sub.m responsible of sound energy from the membrane must satisfy the above relation EQU V.sub.m #V.sub.0 /cos.alpha.
in all the useful audible frequency range. Nevertheless, in order to attempt this relation to be satisfied, it is necessary as much as possible to reduce the reflected waves at the large end of the membrane in such a manner that these waves do not disturb the acoustic radiation produced by the incident waves from the small end of the membrane which is adjacent to the rear portion supporting the voice coil of the electro-mechanical motor of the transducer. In this connection, Lincoln Walsh proposes the vibrations in the membrane to be absorbed gradually from the smaller end to the larger end of the membrane by means of absorbing means such as felt or an elastomeric material which is placed inside the conical membrane, and by means of a very flexible ring-shaped suspension which connects the larger front end of the membrane to the chassis of the loudspeaker. This suspension produces a free axial displacement of the membrane as in the conventional loudspeakers and serves substantially to absorb the vibrating energy for a wide frequency band. This absorption is increased by internal absorbing means which enables also the backward radiated sound waves to be delayed. Additionally, according to other embodiments, in order to weaken the reflection of vibrations at the larger end of the membrane or, in other words, in order that the membrane operates in an analogous manner to an electrical transmission line having a voltage standing wave ratio (VSWR) close to unity (low reflections), the ring-shaped suspension may be constituted by a ring enclosure made of elastomer in which the larger end of the membrane is immersed in a fluid proper to absorb the vibrations. This enables also the load problems of the absorbing means according to the first embodiment to be reduced.
In all cases, although the loudspeaker in accordance with the above-mentioned patent has an acoustical output substantially greater than that of conventional loudspeakers, a non-negligible quantity of the energy of vibrations travelled the membrane is reflected back on the larger end for all the audible frequency range. This occurs mainly from the fact that the vibrations or rattling propagated in the membrane and responsible for the sound energy are transverse waves which belongs to the micro-strain field and act upon the membrane material by shearing. The velocity V.sub.T of these shear transverse waves is given by the relationship: ##EQU1## where G is the shearing elasticity modulus and .rho. is the mass per volume unit. Further, for the usual dimensions of the membranes of loudspeakers, it is not possible to obtain transverse waves without correlating with longitudinal waves which are not responsible for sound emission in most cases and act upon the material by compression-expansion. Since the velocity of these waves is comparatively high, such as approximately 5,100 m/s for longitudinal waves and 2,800 m/s for transverse waves in aluminium, a small energy quantity of these waves during their travel up to the front larger end of the membrane is delivered to air in form of sound waves. It follows from this that the above means for absorbing vibrating energy are required in the membrane and particularly at its front end. Additionally to the drawback relative to the cost increase of the loudspeaker caused by the use of absorbing means, the loudspeakers according to the above-mentioned Patent have the disavantages of the conventional loudspeakers among which I will mention a very low acoustical output and a poor acoustical matching of the membrane in all the usable frequency range since the membrane impedance contains an inductive coefficient again.